Resistance welding is a known process whereby electrical current is passed between two electrodes. Work pieces to be joined, usually made of metallic pieces, are placed between the electrodes, a clamping force is applied to the electrodes, and current flowing between the electrodes passes through the work pieces. The electrical resistance in and between the work pieces, when exposed to the current, creates heat sufficient to locally melt the work pieces. The molten portions of the work pieces are allowed to cool, creating a welded joint between the work pieces.
Spot welding is a known type of resistance welding. It frequently involves two substantially circular copper electrodes that come close to each other, leaving a gap for work pieces to be inserted therebetween. As current is applied, the circular electrodes create a circular weld pattern to the work pieces. Common techniques utilize a series of spot welds to join pieces along a flange.
Other types of, or combinations using, resistance welding are known. Weld-bonding is one technique wherein resistance welding is combined with a structural adhesive to complementarily affix one component to another. Projection welding is another technique wherein a projection is utilized to, at least initially, localize the current flow between work pieces in order to create precise weld geometries and reduce the impact of welding temperatures on the rest of the work pieces away from the joint.
Joint strength or the ability of the joint to experience stress is a property which can be affected by many factors. Stress experienced by two work pieces at a welded joint can take many forms, including axial stress, shear stress, torsional stress, and bending moment. The relationship of stress to strain and the resulting predictable failure for a work piece of known geometry and material properties is well known in the art. Factors influencing joint strength include weld geometry, weld spacing, weld quality, and properties of the work pieces such as material composition and thickness. The appropriate locations and properties of welds along a joint may be determined experimentally, empirically, predictively, through modeling or other techniques adequate to accurately predict reactions of the work pieces to the application of stress. However, variability in welding processes can cause weld locations to change from one set of work pieces to the next. Weld locations affect weld strength and weld quality, impacting how a force applied to the work pieces translates into stresses and resulting strains upon the welds. As a result, weld location variability is important to joint strength.